Valves are commonly used in process control systems to control the flow of process fluids. Sliding stem valves (e.g., a gate valve, a globe valve, a diaphragm valve, a pinch valve, etc.) typically have a closure member (e.g., a valve plug) disposed in a fluid path. A valve stem operatively couples the closure member to an actuator that moves the closure member between an open position and a closed position to allow or restrict the flow of fluid between an inlet and an outlet of the valve. Additionally, to provide desired and/or to achieve certain flow characteristics of the fluid, valves often employ a cage that interposes in the fluid path between the inlet and the outlet of the valve. A cage can reduce flow capacity, attenuate noise, and/or reduce or eliminate cavitation. Additionally, a cage typically surrounds the closure member to provide stability, balance, and alignment to the closure member.
However, unwanted leakage may occur between the closure member and the cage if a proper seal is not employed. Such unwanted leakage may affect the shut-off classification of a valve. For example, the American National Standards Institute has established various leakage classifications (e.g., Class I, II, III, etc.) relating to the amount of fluid allowed to pass through a valve when the valve is in a closed position.
To provide a seal between a cage and a closure member, the closure member typically includes a channel or groove that receives a seal and/or piston ring that engages an inner surface of the cage. The inner surface of the cage often includes a surface finish to provide a sealing surface for the seal and/or piston ring when engaged to the inner surface of the cage. However, such a surface finish increases manufacturing complexity and, thus, increases costs. Typically, the size of the valve and industrial process conditions such as pressure and temperature (e.g., temperatures greater than 600° F.) of the process fluids are used to determine the type of valve and valve components that may be used such as, for example, the types of seals that may be used to effect a seal between a cage and a closure member.
Additionally or alternatively, the pressure differential of the fluid between the inlet and the outlet of the valve may be used to pressure assist or load the seals against a sealing surface (i.e., the walls formed by the groove of the closure member) to effect a seal between the cage and the closure member. However, during low pressure applications, fluid pressure may be insufficient to adequately pressure assist the seal against the sealing surface, thereby causing undesired leakage through the valve.